Technical Field
Embodiments disclosed herein generally relate to propeller driven unmanned aerial vehicles of the type that are powered by electric motors. Specifically, certain embodiments disclosed herein relate to methods and structures which can be used to manage heat dissipation from the electric motors in such vehicles.
Description of the Related Art
The performance and operation of most electronic and electrical systems and devices are adversely affected by heat. Unfortunately, most electronic and electrical systems and devices generate heat as they operate. The generated heat must be managed to prevent a reduction in performance or overall operational failure of such electronic and electrical systems and devices. The electric motor is one such device.
When designing a device that is powered by an electric motor, important considerations include output speed and output torque. As described below, the performance characteristics of two main components of an electric motor change with an increase in temperature, the resistance of the motor's windings and the flux density of the motors permanent magnets. These changes will affect the performance of the motor and, in turn, the performance of the device that the motor powers.
Winding resistance and permanent magnet flux density will change as temperature changes. As the temperature within a motor housing increases, winding resistance will increase based on the temperature coefficient of copper (which is typically used in motor windings). The flux density of the permanent magnets will also decrease as a function of temperature. Changes in these two key components of the motor will result in an increase in motor no-load speed and a decrease in motor locked rotor torque, altering the overall slope of the motor curve.
The motor torque constant and voltage constant are directly related to the magnetic flux density of the permanent magnets. Depending on the physics of the magnet material used, overall flux density will change at a given percentage with an increase in magnet temperature. As the material temperature increases, atomic vibrations cause once-aligned magnetic moments to “randomize” resulting in a decrease in magnetic flux density. Assuming the motor is operating within its intended design window, the decrease in flux density is temporary and will begin to recover as the magnet cools. If the maximum temperature rating of the magnets is exceeded, however, partial demagnetization will occur and permanently alter the performance of the motor.
Motor winding resistance is the main cause of heat generation within the motor. In order for any electric motor to generate torque, current needs to be forced through the motor windings. Copper is an excellent conductor, however, it is not perfect. Material physics and impurities will cause the atoms within the copper to vibrate at a faster rate as more current flows. The result is a steady temperature increase in the motor windings as the motor operates.
Another potential cooling-related failure mode is that motors are commonly designed to include small air gaps between motor components, such as between the stator and the rotor, for maximum motor efficiency. Modern electric motors used in AUVs often have air gaps that are similar in size to a grain of sand, making them vulnerable to having sand or other small particles caught in the air gap, causing the motor to seize.
Management of heat generated within an electric motor can ensure that the motor remains “cool” and therefore maintains operating efficiency and provides acceptable power output.